1. Field of the Invention
The invention relates to the field of monitoring the hydration and/or nutrition status of a patient.
2. Description of the Related Art
The kidneys carry out several functions for maintaining the health of a human body. First, they control the fluid balance by separating any excess fluid from the patient's blood volume. Second, they serve to purify the blood from any waste substances like urea or creatinine. Last not least they also control the levels of certain substances in the blood like electrolytes in order to ensure a healthy and necessary concentration level.
In case of renal failure ingested fluid accumulates in body tissues and the vascular system causing increased stress on the circulatory system. This surplus fluid has to be removed during a dialysis treatment by ultrafiltration of the blood. If insufficient fluid is removed the long term consequences can be severe, leading to high blood pressure and cardiac failure. Cardiac failure itself is many times more likely to occur in dialysis patients and it is thought that states of fluid overload are one of the major contributing factors. Removal of too much fluid is also dangerous since the dialysis patient becomes dehydrated and this invariably leads to hypotension.
The dry weight (for the sake of simplicity the words “weight” and “mass” shall be used synonymously throughout this patent application document—which is usual practise in the medical field anyway) defines the weight of a patient that would be achieved if the kidneys were working normally. In other words this represents the optimal target weight (or fluid status) which should be achieved in order to minimise cardiovascular risk. Dry weight has always been an elusive problem in routine clinical practise due to lack of quantitative methods for its assessment. Currently the dry weight problem is approached using indirect indicators like e.g. blood pressure, echocardiographic investigations and subjective information such as X-rays. Furthermore it has been particularly difficult to define a set of conditions which are universally accepted as the dry weight standard.
A promising method to derive the fluid status of a patient involves the use of bioimpedance measurements. A small alternating current is applied to two or more electrodes which are attached to a patient and the corresponding electric potential difference is measured. The various fluid compartments of a human body contribute differently to the measured signals. The use of multiple frequencies allows the intracellular water (ICV) and extracellular water (ECV) volumes to be determined. An example of such a device is described in the international patent application WO 92/19153. However, this document discloses no method regarding how the dry weight of the particular patient can be derived.
The U.S. Pat. No. 5,449,000 describes a bioimpedance system also using multiple frequencies to determine ECV and ICV. Furthermore certain population dependent data are taken for using and choosing so-called population prediction formulas. The body composition is then analysed by using these formulas and with the help of segmental bioimpedance signals. This document also lacks a disclosure of a method how the dry weight may be derived.
Hence there is a need for a non-invasive, accurate and easy to use method for dry weight assessment which nevertheless takes into account individual variations without grossly limiting the analysis to certain populations. This method would be of major benefit to the management of dialysis patients and could significantly reduce hospitalisation costs in the long term. There is also a need for an easy to carry out method to assess the body composition of a patient in a more general manner, providing additional insight into the nutrition and training status.